Since print systems have been in existence, printers have sought methods for inhibiting counterfeiting and unauthorized copying of printed documents. Enhanced complexity in an engraved pattern of a press plate is one such method that most people are familiar with as a result of its everyday observation in currency bills. Bank checks, security documents, bonds and other financial documents are other examples of printed documents having complex background patterns to inhibit unauthorized reproduction. Identification documents, e.g. passports, social security cards and the like, are other examples. Credit cards not only have complex background patterns, but now also have embedded holographics to enhance verification and authentication of such a card.
It is desirable to have a way to provide for the detection of counterfeiting, illegal alteration, and/or copying of a document, most desirably in a manner that will provide document security and which is also applicable for digitally generated documents. It is desirable that such a solution also have minimum impact on system overhead requirements as well as minimal storage requirements in a digital processing and printing environment. Additionally, it is particularly desirable that this solution be obtained without physical modification to the printing device and without the need for costly special materials and media.
Watermarking is a common way to ensure security in digital documents. Many watermarking approaches exist with different trade-offs in cost, fragility, robustness, etc. One prior art approach is to use special ink rendering where the inks are invisible under standard illumination. These inks normally respond to light outside the visible range and thereby may be made visible. Examples of such extra-spectral techniques are UV (ultra-violet) and IR (infrared). This traditional approach is to render the encoded data with special inks that are not visible under normal light, but have strong distinguishing characteristics under the special spectral illumination. Determination of the presence or absence of such encoding may be thereby subsequently performed using an appropriate light source and detector. One example of this approach is found in U.S. Patent Application Publication No. US-2007-0017990 to Katsurabayashi et al., which is herein incorporated by reference in its entirety for its teachings. However, these special inks and materials are often difficult to incorporate into standard electro-photographic or other non-impact printing systems like solid ink printers, either due to cost, availability or physical/chemical properties. This in turn discourages their use in variable data printing arrangements, such as for redeemable coupons or other personalized printed media for example.
Another approach is digital watermarking, as for example U.S. Pat. No. 5,734,752 to Knox, where there is provided a method for generating data encoding in the form of a watermark in a digitally reproducible document which is substantially invisible when viewed including the steps of: (1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document; (2) deriving at least one stochastic screen description that is related to said first pattern; (3) producing a document containing the first stochastic screen; (4) producing a second document containing one or more of the stochastic screens in combination, whereby upon placing the first and second document in superposition relationship to allow viewing of both documents together, correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes visible.
U.S. Patent Application Publication No. US-2008-0302263-A1 discloses a system for creating an infrared mark employing the different infrared transmission characteristics of standard non-impact printing materials, specifically the different infrared transmission characteristics of the four (CMYK) or more printing colorants, whereby the application of such infrared transparent colorants on a substrate results in a high level of infrared reflectance of the combination due to the substrate reflectance characteristics. The infrared mark is created by printing the first colorant combination with a relatively high infrared reflectance in direct spatial proximity to a second colorant combination having the essentially same visual response under visible light, while having a different infrared reflectance by changing the relative amounts of the colorants in the mixture in a manner that is essentially invisible to the human eye under normal illumination.
The underlying principal is that carbon-base black toner is a good absorber of infrared light, while the other toners (e.g., cyan, magenta and yellow) are almost transparent in the infrared region. This infrared watermark and distracting background are rendered with two different GCR (Gray Component Replacement) strategies that either maximize or minimize the usage of black toner. This system is limited in that the watermark critically depends on the accurate printer characterization and might be perceptible due to the poor printer characterization or the printer drift from time-to-time. In other words, the high IR absorption component (K) color of the mark may not match the low absorption component (CMV) color under normal light conditions so that the document is not acceptable for practical or commercial use.
Watermarking in frequency domain is also known and usually involves alternating frequency components according to watermarking messages. Such techniques result in a watermark that is dispersed throughout the image. However, all such known methods are based on visible lights and so can be human perceptible as well as machine.
More particularly, there is a need for a system which is based on IR detectable security information that has advantages over conventional frequency domain watermarking in two main aspects: (1) it easily enables “blind decoding”, more specifically, the original image is not necessarily required in the decoding process and as a result, the method can be used more broadly in many applications, and (2) the method is less noisy. In conventional methods, the watermark noise is one of the limiting factors in design consideration. The desired method would conceptually reach zero (visual) noise.
There is thus a need for a system which better hides an infrared watermark within a printed document without being subject to system temporal change.